We propose a novel approach to investigate in the chick embryo the biochemical mechanisms of cells about to undergo determination: using temperature-sensitive mutants of transforming viruses, such as Avian Erythroblastosis (AEV) and Rous Sarcoma (RSV) viruses, we will arrest reversibly the development of precursor cells in primary mesenchyme from chick embryos at 24 hours of incubation or earlier. Infection of prestreak epiblast cells is also planned. This transformation will cause the precursor cells to proliferate greatly, thus making them available for cloning and for biochemical investigations. In each case mass cultures and clones will be derived and their biochemical characteristics will be stuided. Using AEV and AEV-ts34 or RSV and RSV-ts68, the timing of turning-on of the production of different globin chains will be investigated. We intend also to search for clones of transformed multi-potent precursor cells by using the appropriate immunofluorescent probes for celltype-specific protein products. The biochemical mechanism whereby virus blocks globin and heme synthesis will be investigated with particular attention to globin mRNA metabolism. The possible role of temperature-sensitive protein kinases in AEV-ts34 and in RSV-ts68 transformed clones will also be studied. In the quail embryo similar infection and transformation of primary mesenchyme tissue is planned, using both AEV and RSV, because it is known that in the quail permanent cell lines are more easily established.